Ovipositors... A Closer Look
According to BiologyDictionary.net, ovipositors are appendages composed of hardened sclerites of the exoskeleton that are used for placing and transmitting the egg. This hardened tube structure is also used for piercing the host, or wood in the case of Megarhyssa macrurus. Speaking of M. macrurus, their slender ovipositor has the title of the longest egg laying organ to body ratio known to biologists!
Ovipositors have 3 main functions in Ichneumons; releasing mature eggs, piercing, and counting.
By the time the egg reaches the ovipositor, it is fully mature and ready for deposition. Muscle contraction moves the egg down the ovipositor and into or on the host. This miracle of life takes a dark turn for the host as was mentioned in the Life Cycle post.
Ichneumons usually have to pierce the exoskeleton of a host or go through wood or other organic matter. Long, thin ovipositors with jagged edges make this task easier. Because the ovipositor is so thin, the egg increases in size when it is deposited.
Multiple Ichneumons can parasitize a single host, and with more eggs is more competition for resources and this could prematurely kill the host. To prevent this from happening, Ichneumon ovipositors are capable of detecting how many foreign eggs are already in a host. The ovipositor is equipped with sensory receptors at it's tip that connect to neurons. These receptors sense pheromones on eggs laid by another parasitoid when the ovipositor is penetrated. The neuron is triggered when high pheromone concentration is detection, which allows the Ichneumon to count the number of foreign eggs.
Figure 1: Phylogenetic tree proposing the evolutionary adaptation of stingers in Hymenoptera from the homologous structure of the ovipositor.
It is known that ovipositors are conserved structures across all members of Hymenoptera that have been altered over time to serve a different function.
Aculeata is the suborder of Hymenoptera in which the ovipositor was modified and associated with venom glands for the purpose of paralyzing it's prey. In figure 1, the modification of the ovipositor can be seen as trait 2, and the loss of function of the ovipositor as an egg laying organ is trait 1-0. Note that only one family has both the ovipositor and the functional stinger (Chrysididae). It is thought the association with venom glands introduced a defensive weapon against predators and prey increasing the fitness of these species.
The loss of function of the ovipositor as an egg laying organ is thought to have been due to social pressure from the species. When only the queen is responsible for laying eggs, the energy required to produce eggs is selected against, leading to the loss of function.
It is also important to note the reduction of the stinger in many species, for example the family Apoidae. With a diet composed of pollen and nectar, the stinger is only used for defensive purposes at close range, so the long ovipositors are no longer necessary as they require too much energy to grow and support.